This invention relates in general to sewing machines and in particular to a new and useful sewing machine feed mechanism which operates with very low inertia.
A sewing mechanism similar to the present invention is disclosed in U.S. Pat. No. 3,742,879. That prior art feed mechanism comprises two swing arms designated X and Y, which are mounted on fixed bolts and carry each a stepping motor as a positioning drive. The stepping motor secured to the Y arm drives through a pinion a geared rack which is mounted for displacement in the Y arm and hinged to the X arm. The stepping motor secured to the X arm drives, through the pinion, a geared rack which is mounted for displacement in the X arm and to which a work holder is secured. Even though the two-member drive linkage of this reference reduces the number of component parts to be moved, as compared to a prior art four member drive linkage, known for example from U.S. Pat. No. 3,983,845, inertia of this mechanism is still relatively too high since the stepping motors are secured to the swing arms and therefore are moved along with the arms.
A low inertia mechanism for driving a fabric clamp comprising a single sewing arm is known from U.S. Pat. No. 3,974,787. This is a telescopic structure where a slide is mounted in a swing carrier. Two stationary stepping motors drive the swing carrier and the slide by means of two ropes trained about rollers which are partly fixed and partly carried on the swing arm. The advantageous low inertia is outweighed by the disadvantage that this drive system is suitable only for small seam patterns, thus with a large radial displacement of the swing arm, a retracted swing arm produces small angular increments of the fabric clamp per step of the driving motor, while an extended swing arm causes large such increments. A relatively expensive stepping motor system would therefore be needed for driving the swing carrier, to obtain a highly accurate and fast feed, namely a system with small steps and a large stepping frequency.
Among other drawbacks, the ropes may become permanently extended, for example due to the material fatigue with the result of transmitting the motor steps inaccurately, or the drive system may oscillate at certain frequencies because of the shock absorbing springs provided between the machine frame and the stepping motors.